Combined frequency modulation and amplitude modulation detector circuits



oct. 28, 1.947. w. R. KOCH COMBINED` FREQUENCY MODULATION AND AMPLITUDE MODULATION DETECTOR `CIRCIUITS 2 Sheets-Sheet l Filed Fgb. 5. 1944 ddl/70 HTLV Qu s .hit Ok iuHrrVl INVENToR. wmf/nn 1. Ko o# Armi/vakI W. R. KOCH AooMbuitD FREQUENCY MODULTION AND AMPLITUDE MODULATION'DETECTOR cIRcUITs 2 Sheets-Sheet2 -Filed Feb. 5. 1944 f INI/ENTOR. Wim-fab E. /fbcH 4 BY i frog/vsn Patented Oct. 28,u 1947 COMBINED FREQUENCY MODULATION AND AMPLITUDE MODULATION DETECTOR CIRCUITS Winfield R. Koch, Haddoneld, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application February 5, 1944, Serial No. 521,193

(Cl. Z50- 27) Claims.

My present invention relates to receivers of frequency modulated (FM) or amplitude modulated (AM) carrier waves, and more particularly to novel and improved receivers of FM or AM carrier waves utilizing switchless demodulator or detector circuits.

In his U. S. Patent No. 2,296,092, granted September 15, 1942, M. G. Crosby has disclosed a differential detector circuit adapted to receive FM and/or AM carrier waves. In his detector circuit Crosby provides automatic volume control (AVC) Voltage at the detector output in response to either FM or AM reception. The AVC voltage is derived from the separate rectified voltages of the balanced rectifiers added in aiding phase, while the modulation signal corresponding to the frequency modulation of a carrier wave is derived from the rectied voltages added in phase opposition.

While Crosby showed a common signal input network for feeding the FM or AM signals to the detector, it is often desirable to provide separate and independent FM and AM signal channels to the detector and yet be able to provide the same output voltages as in the Crosby system'. Accordingly, it is one of the principal objects of my invention to provide discrimination and detection for FM reception on -one frequency channel preferably with additive AVC voltage, and detection for AM reception on a different frequency channel without switching of any kind in the output of the detection stage.

When receiving FM signals with a receiver having a substantially flat-topped selectivity characteristic and no amplitude limiter is used, it is desirable to provide AVC voltage from the opposed diodes of the discriminator-detector circuit. Again, where such a receiver is provided with a separate signal channel for AM broadcast reception, it is desirable to use one of the opposed diodes for AM detection and AVC recti-cation without switching. It is an important object of my invention to provide such an FM-AM receiver with minimum circuit .components and maximum detected voltage output.

Other objects of my present invention are to improve generally the elciency of FM-AM receivers, and more especially to provide economical detector circuits for such receivers,

Other features will best be understood by reierence to the following description, taken in `connection with the drawing, in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

2 In the drawing: Fig. 1 shows, in partial schematic form, an

FM-AM receiver employing one embodiment of my invention,

Fig. 1a shows an ideal selectivity characteristic of the FM signal channel,

Fig. 1b shows an ideal selectivity characteristic of the AM signal channel, Y

Fig. 1c illustrates the FM detection characteristic,

Fig. 1d shows the frequency-AVC voltage characteristic for FM reception,

Figs. 2, 3 -and 4 show respectively different modifications of the detector network of Fig. 1.

Referring now to the accompanying drawings, wherein like reference characters in the different iigures designate similar circuit elements, Fig. 1 shows an illustrative receiving system embodying a demodulator network adapted to provide audio voltage and AVC voltage in response to FM or AM signal reception. The receiver circuits prior to the demodulator are schematically represented. Those skilled in the art of radio reception are well acquainted with the nature of the circuits .customarily employed in multi-band receivers. While my invention is readily adapted for FM and AM reception on respective bands of 42 to 50 megacycles (mc.) and 550 to 1700 kilocycles (kc), it is to be clearly understood that the invention is not limited to such frequency bands. The 42 to 50 mc. band is presented by way of illustration, since it is the FM broadcast band presently assigned to such transmission. The 550 to 1700 kc. band is the present AM broadcast band assigned to transmission of AM signals.

It will further be understood that in the following description and Iclaims the generic expression angle modul-ated is intended to include frequency modulation, phase modulation or hybrid modulations possessing characteristics common to either form of modulation. From a very general viewpoint my invention relates to a demodulator network having separate input circuits for carrier waves of different frequencies and of different modulation characteristics.

The numerals I and 2 in Fig. 1 denote respectively dilferent sources of modulated carrier waves. Source I. may be the usual signal collector, such as a dipole, employed for collecting FM waves. The FM waves are transmitted from FM transmitters at a mean, center or carrier frequency assigned to the particular transmitter. In the assumed FM band of 4 2 to 50 mc. the radiated carrier Wave frequency would be in that range, and would be a wave of variable frequency and substantially uniform amplitude. As is well known, the frequency modulation of the carrier wave would be in accordance with the modulation signals at the transmitter. The extent of frequency deviation of the carrier frequency is a function of the modulation vsignal amplitude, While the rate of frequency deviation is dependent upon the modulation signal frequencies per se. The permissible eXtreme frequency deviation in the FM band of 42 to 50 mc. `is '75 kc. to either side of the carrier frequency; the allotted vFM channels are 200 kc. Wide. 'Ihese values are purely illustrative.

Source 2 may be the customary grounded antenna circuit employed in AM broadcast reception. The allotted channels are kc. wide in this band. In AM transmission the carrier wave .ismodulated in amplitude in accordance with the modulation signals. The carrier 'frequency is maintained constant in value at the transmitter. The numeral 3 designates a tunable radio frequency amplifier having suitable signal selector .circuits for FM or AM reception. Switching devices 4 .and 5 respectively provide separate connection of :the sources I and 2 to respective selector circuits .of amplifier 3. It will be Vunderstood vthat when switch 4 is in closed position, col-lected FM signal energy will b'e applied to selector circuits .of amplifier 3 capable of selectively amplifying 'the .FM signals over a band at least 150 kc. wide. Upon closing of switch 5 and opening switch 4 thesame amplifier 3 will have the FM selector circuits thereof operatively replaced by AM selector circuits. These latter circuits will Vselect the collected AM signals and permit amplifier E to lvarnplify the same over a ,10 kc. band. Multi-band selector circuits and switching devices for suitable change-over are well known to those vskilled .in the art of radio communication. Switching `devices l and 5 affect the demodulator circuit only in so far as they determine the character of the modulated Wave to be delivered tothe demodulator.

Assuming the system is of the superheterodyne type, as is the usual practice at present, the converter 5 and intermediate frequency (I. F.) amplifier 'I will also be provided with `suitable FM and AM signal selector circuits. At the converter 6 the FM signals will have the mean or center frequency thereof reduced toa value which `may be chosen from a range of .l to `zmc., as for example 4.3 mc. 'I'he AM signals are reduced to an I. F. of 455 kc., as an illustrative frequency value, the latter being a commonly employed frequency in AM broadcast receivers of the superheterodyne type. 'Ihe I. F. amplifier l, which may consist of one or more `separate stages of amplification, will have an ultimate output circuit from which may `be derived, ya-t .separate points thereof, the amplified FM signals or AM signals.

The selective circuits 8 and 9 `are to be understood as being arranged in series `in the plate circuit of the last I. F. amplifier tube. Eachof circuits 8 and 9 is resonated to its respective operating I. F. value for FM or AM reception. Thus, circuit 6 is tuned to 4.3 mc., while circuit 9 is tuned to 455 kc. There will be developed across tuned circuit 8 the FM signals at the 4.3 mc. mean frequency when switch 4 is closed, and all FM selector circuits of amplier 3, converter VI5 and I. F. amplifier l are in operative electrical connection. Conversely, when switch 5 is closed, and switch 4 is open, and all AM selector vcircuits are in operative electricalV c,orln.ectif.lr1,l .there will atively coupled to the circuit 8.

be developed across circuit 9 AM signals at the I. F. value of 455 kc. The impedance of circuit 9 is negligible at 4.3 mc.; therefore, the insertion of circuit 9 in series with circuit 8 will not affect the development of FM signal voltage across circuit 8. Similarly, the impedance of circuit 3 is negligib'leat 455 kc., and circuit 8 'will not affect development of AM signal voltage across circuit 9.

A'The demodulator comprises but two electron ydischarge devices, shown as diodes by way of illustration. The electrodes of the pair of diodes may be housed within a common tube envelope, or they may be in separate envelopes. By way of specific illustration the diodes Ill and I I are shown as being'separatetubes. The diode IIJ is provided with a resonant input circuit I2 which is induc- 'Ihe anode of diode I!! is connected to one side of the input circuit I2, while the cathode of diode I0 is connected to the opposite side of input circuit I2 through the load resisto-r 13. Resistor I3 is bypassed by condenser If4 for high frequency currents.

Diode I.I ihas :its cathode established at ground potential, while its anode is connected to the high alternating potential side of its resonant input circuit I5. Circuit "I5 is also inductively coupled to the circuit 8. The low potential sideof circuit I5 is connected to groundxthrough .the coil I'I and load resistor Il. Coil I6 yis magnetically coupled to circuit i9, and condenser SIB shunts coil yI6 to provide a resonantcircuit I-lituned to 455 kc. Condenser .I9 shunts .resistor Ill 'to bypass high frequency currents.

The input circuits I2 `and :I5 of diodes ".I and I1I respectively are oppositely and equally mis tuned with respect 'to the yoperating I. F. value for FM reception. In other words, if the FM signals developed across circuit 8 have .a center frequency of Fc (4.3 mc), then `circuits I=2 and I5 will `be detuned vin opposite senses :by equal lpredetermined frequency values relative to Fc. It will be recognized that circuits 8, I2 and VI5 provide the well known discriminator Ynetwork of Conrad U. S. Patent No. 2,057,640. The .action'of this form of discriminator rcircuit is well known to those skilled `in the art. It'functions zto ytranslate FM `high frequency signals into'corresponding AM high frequency signals.

At the 4.3 mc. frequency used for FM reception the limpedance of circuit ILS-I8 is negligible, and hence the load resistor I'I 'is effectively in series with input circuit I5 and diode II. The upper end of resistor I'I is coupled to .the lower end of resistor 1I 3 through the'conden'ser 29. Condenser ZI! has a low impedance for the modulation frequencies developed during detection of the FM signals, but has a high impedance to high frequency currents. In other words, condenser 2S is a modulation frequency coupling condenser.

Assuming that the modulation signals on the received FM :waves are of audio frequency, then the audio frequency amplifier ofthe receiver will have its inputlead connected tothe cathode end of load resistor I3. As already known, a de-emphasis network 2| lmay be employed in the audio frequency output connection in order to cornpensate for pre-emphasisfof higher audio frequencies at the transmitter. I-n accordance with wellunderstood principles of FM signal detection, the alternating current components in the rectified signal voltageacrosseach `0f resistors I1 and I 3 will becombined in phase opposition due to the connectionof the anode 4endof resistor I'I to the anode end of resistor I3 by coupling condenser 20..

The differential voltage resulting from the phase-opposing voltages corresponds to the audio modulation signal voltage originally applied to the FM carrier. wave at the FM transmitter. At the same time there is provided a conductive connection between the negative or anode end of re-Y sistor I'I and the cathoder or positive end of resistor I3. This conductive connection includes resistor 22.' Considered relative to ground the direct current voltage components of the rectified voltages appearing across resistors I3 and I1 are added in phase-aiding sense.' In other words, the direct current voltage components of rectiers I and II are combined in additive manner during FM signal reception, while the alternating current (audio) output of rectiers I and II are combined in phase-opposing relation.

An AVC connection 23 is provided between the gain control electrodes, as for example the signals grids, of the various tubes in networks 3, 6 and 'I and the negative end of resistor I3. The AVC connection 23 includes a filter resistor 24, whose lower end is bypassed to ground by an audio frequency condenser 25, so as to prevent alternating current components from being transmitted over the connection 23. Network 24-25 therefore acts as a time constant network to produce slow AVC action.

The function of the AVC connection is well known to those skilled in the art. Should there be any carrier amplitude variation at the input terminals of each of rectiers I0 and II, such amplitude variation will be translated into a corresponding change in direct current voltage across the corresponding load resistors I3 and I1. The additive AVC voltage applied over connection 23 to the controlled tubes will reduce the gain of the tubes to counteract undesired carrier arnplitude increase.

During AM signal reception the I. F. signal energy produced in the circuit 9 will be transferred to input circuit Iii-i8. Each of circuits 9 and Iii- I8 is tuned to the operating I. F. value of 455 The circuit I and diode I I are in a series circuit with tuned circuit I5-I8 and load resistor I 1. The circuit I5, resonant close to 4.3 mc., has no appreciable effect on the series circuit, since it acts as an extremely low impedance connection at the 455 kc. value. The modulation voltage component of the rectied I. F. energy developed across bypassed load resistor Il is applied through condenser 23 and resistor I3 to the common modulation signal output circuit. The direct current voltage component across resistor I'I is applied over AVC path 23 to the prior tubes. The AVC line 23 connects to the ungrounded end of resistor Il, through a series path consisting of resistor 24, resistor I3 and resistor 22. Here, again, the network 'Z4- 25 acts to introduce time delay into the AVC action.

In Fig. 1a I have shown the form of selectivity characteristic which is preferred for use during FM signal reception. The curve is idealized, and represents a flat-toppedcharacteristic at least 150 kc. wide. The characteristic represents the ideal pass band of the receiver circuits up to the opposed rectiers i--II during FM reception. The nat-topped selectivity characteristic, if the FM carrier is correctly centered on it, insures against production of amplitude modulation on the FM wave as the latter passes through the cascaded resonant circuits to the FM detector circuit, and lessens the Vimportance of the use of an amplitude limiter stage in FM reception. The AVC circuit acts effectively to reduce the gain of the receiver tubes in response to increases in amplitude of the FM carrier.

By way of contrast to Fig. 1a I have shown the AM selectivity characteristic in Fig. lb. This curve is idealized, and represents the nat-topped 10 kc. pass band of the receiver circuits up to the rectier during/AM signal reception. This en-v ables faithful AM reception and permits the AVC action to function in the well understood manner. Fig. 1c shows the FM detection characteristic of opposed rectiflers I0 and II and their associated input circuits I2 and I5. It is desirable to have the spaced peaks of the ideal curve separated by a frequency value in excess of the 150 kc. band width. Further, the curve should be as linear as possible between the peaks thereof. With a detection characteristic as shown in Fig. 1c, the AVC voltage-frequency characteristic during FM reception will be substantially of the form ideally represented in Fig. 1d. It will be noted that the AVC (negative in sign) Voltage, with changes in carrier frequency but not in amplitude, becomes a maximum at spaced peaks of the curve, with a decrease towards the center frequency Fc. In other words, the AVC bias will be a Vmaximum on each side of Fc thereby providing an audible aid in differentiation between exact tuning of the receiver and off-center tuning thereof. I

The detection network of Fig. 1 may assume different forms. Figs. 2 to 5 inclusive show different modifications of the invention. In Fig. 2 the rectier diode I0 has its cathode connected to the high potential side of input circuit I2, while the anode of the diode is .connected to the upper end of load resistor I3, which is bypassed by condenser I4 for high frequency currents. Condenser I3' bypasses the anode end of resist-0rV I3 to ground for both I. F. and A. F. currents. The anode of diode II is connected to v the grounded cathode through a series path comprising circuit l5, circuit lli-I8 and load resistor I '1. The cathode end of resistor I3 is connected directly to the upper end of resistor 26, while the lower end of the latter is connected directly to the ungrounded end of load resistor I1. The modulation or audio output connection is made to an intermediate point on resistor 25. The AVC connection 23 is made to the anode end of resistor I3 through the time constant resistor 24.

The action of Fig. 2 is similar to that of Fig. 1. The AVC bias, during FM reception, is secured by additively combining the rectified direct current voltages across resistors I'I and I3 which are connected through resistor 26. The audio output voltage during FM reception is produced by combining in phase opposition the rectied voltages produced at audio frequency across resistors I 3 and II. The resultant audio voltage appearing between the center tap of resistor 26 and ground is used. For AM reception the diode II acts as the rectier whose input circuit is IS-IB. The audio voltage across resistor I'I is supplied to the audio network through resistor 2", while the AVC line 23 is connected to the negative end of resistor I1 through the path comprising resistor 24, resistor I3 and resistor 26.

In order to explain the function of centertapped resistor 26 in this combination of ele ments, there will be considered what happens when signal amplitude changes on the one hand, and what happens due to signal frequency changes on the other hand. As the signal amplitude becomes larger with the frequency remaining constant, the end X of resistor 2t becomes more positive relative to ground, it `being `understood that the anode end of resistor I3 is grounded for audio frequencies through condenser I3'. The other end Y of resistor 25 becomes more negative. Both of these effects follow from the direction of yconnection of diodes I8 and I I. The center tap of the resistor therefore remains at ground potential despite amplitude changes. This satises the requirement of a discrrninator that it be balanced against amplitude modulation eiects for a carrier tuned to the center of the band. As the signal frequency changes in one direction, however, the end X of resistor 26 becomes less positive while the other end Y becomes more negative. Hence, the center tap on the resistor assumes a negative voltage relative to ground. For changes in frequency of the carrier in the other direction, the end X of resistor 26 will become more positive and the other end Y less negative thereby causing the center tap to be positive, These last-mentioned changes occur at the audio modulation frequencies. This satisfies la second requirement of a discriminator that the output thereof shall follow frequency variation so as to derive the modulation frequencies. In the present discriminator circuit the outputs of the diodes are combined by approximately a parallel connection between the mid-point of resistor 25 and ground, which permits` one side of each diode load resistor to be at ground potential for audio frequencies as Well as for intermediate frequencies.

In the modification of Fig. 3 the AM circuits 9 and Iii-I3 are connected in common at the lower sides thereof. In place of the FM circuit 8, only the oli-tuned circuits I2 and I5 are ernployed. These circuits I2 and I5 correspond to circuits I2 and I5 of Fig. l. Circuit l2 tuned to a frequency somewhat above Fc is arranged in series with circuit I tuned to a frequency less than Fc by the same amount. The circuits 9, I2' and I5 are arranged in series in the I. F. am.- plier output circuit. Diode ii is arranged in series with circuits IE5-Iii and I2', the cathode of diode II being connected by radio frequency coupling condenser 2'? to the high potential side of circuit Iii-i8. Condenser 2S, an audio coupling condenser, connects the anode of diode il to the junction of circuits I2 and l5. Load resistor Il shunts the diode II. Diode Il) has its cathode grounded, while its anode is connected by radio frequency coupling condenser 29 to the junction of circuits i2' and I5. The audio coupling condenser 33 connects the lower side of circuit I5' to the grounded cathode of diode I0. Load resistor I3 shunts the diode I0. The AVC connection 23 is made through time constant resistor 2t, associated with condenser 25, to the anode, or negative, end of resistor I 1. The tapped resistor 2G, from whose mid-tap the audio voltage is taken, connects the cathode end of resistor `Il to the anode end of resistor I3.

As in the previous modifications, rectier l I has AM signals applied thereto through the circuits 9 and Iii-I8. The circuit I2 acts as a low impedance connection relative to AM signal frequencies. The AVC bias is taken from the resistor Il, while audio voltage is taken from resistor 25. For FM reception circuit 3 has negligible impedance. Circuit I2 acts as the input circuit of diode Il, while circuit I5 functions to supply FM energy to diode iii. The action of the series discriminator I2-E5 in Fig, 3 is similar to that of Figs. l and 2 save that the primary circuit B is eliminated. The direct current voltages across 8, resistors Il' and I3 are added in phase aiding relation by connection through resistor 26, and the AVC connection 23 employs the additive Voltage. The audio voltage components across resistors I'I and I3 are combined in opposition across resistor 26 in the same manner as in the circuits of Fig. 2. Condensers 28 and 30 of Fig. 3 ground the anode end of resistor I1 for laudio frequencies in like manner to the action of condenser I3 in Fig. 2 in grounding the anode end of resistor I3.

The modication of Fig. 4 employs the principles of operation of Fig. 2, but differs therefrom in respects which adapt the audio output circuit of the network for push-pull operation. In the modication of Fig. 4, each of resistors I3 and Il acts as the respective load resistor for rectiers I0 and II during reception of FM signals. Furthermore, resistor I'I is the load resistor for rectifier I I during reception of -AM signals. The AVC connection 23 is made through filter resistor 24 to the yanode end of resistor II. Condenser 25 cooperates with resistor 2li to provide the time constant network for the AVC circuit. It will be seen that the direct current voltages developed across resistors I'I and I3 are additively combined, since the connection 23 is made to the negative end of resistor ITI, and the positive end of the latter resistor is connected to the negative end of resistor I3 through resistor 3l. The positive or cathode end of resistor I3 is connected to ground through a path including r8- sistors 32 and 33 in series. The mid-taps on resistors 3l and 32 may act las opposed-phase input terminals for a following push-pull audio amplifier. The mid-tap on each of load resistors I3 and Il is connected to ground` through respective audio frequency bypass Condensers III and In a manner similar to the operation of resistor 26 in Fig. 2, a connection may be traced from the mid-tap of resistor 32 to ground through the upper end of resistor 32, upper half of load resistor I3, and condenser Illl, and a second connection in parallel therewith may be traced from the mid-tap of resistor 32 to ground through the lower end of resistor Y32, condenser 25, lower half of load resistor II and condenser 02. These connections will cause the potential at the midtap of resistor 32 to change with variations in the signal frequency in the same manner in which the potential of the mid-tap of resistor 26 changes in the operation of Fig. 2. In like manner two parallel connections are established between the mid-tap of resistor 3l and ground, one through the upper half of resistor 3l, lower half of resistor I3, and condenser ISI and the other through the lower half of resistor 3 I, upper half of resistor I'I and lcondenser |82. It will be apparent to those skilled in the art without further explanation that the potential of the mid-tap of resistor 3l also changes with variations in the signal frequency, and in opposite phase to the potentials on the mid-tap of resistor 32. The Condensers IUI and |02 may also act to improve the response at the higher audio frequencies by providing a good path to ground for such frequencies. .During AM signal reception, it being understood that circuit I5 has negligible impedance, rectifier II is fed with AM signals by input circuit I6--I8. The direct current voltage `developed across resistor Il is ernployed for AVC bias, whereas the audio voltage across resistor I 'I is used for push-pull operation since the input terminals, or the center- `{ifi-QS oii resistors 3| and 32, are connected to points of ,opposite polarity on resistor Il relative to audio frequency ground at condenser |02.

While I have. indicated and described several systems for carrying my invention into eifect, it Will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be m-ade Without departing from the scope of my invention.

What I claim is:

1. In combination with a frequency discriminator, a pair of opposed rectiers each having an input electrode coupled to said discriminator, a separate load impedance in circuit with each rectier, resistive means connecting said 4load impedances in series relation, means grounding the positive potential end of one impedance a rst connection to the negative potential end of the series-connected second impedance for deriving from said impedances in additive polarity sense rectified voltages thereacross thereby to provide a negative voltage relative to ground, a second connection to an intermediate point on said resistive means for deriving a voltage resulting from said rectified voltages being combined in polarity opposition.

2. In combination with a frequency discriminator, a pair of opposed rectifiers each having an input electrode coupled to said discriminator, a separate load impedance in circuit with each rectier, a first resistor connecting a first pair of opposite polarity points of said load impedances,

a second resistor connecting a second pair of opposite polarity points of said load impedances in reversed sense relative to the rst resistor, a rst connection for deriving from said impedances in additive polarity sense direct vcurrent voltages thereacross, respective output connections t an intermediate point on said resistor means for deriving a push-pull modulation signal voltage resulting from rectified voltages being combined in opposition, a source of amplitude modulated carrier Wave energy coupled to one of the rectii'lers and including its respective load impedance in circuit therewith, and said output connections deriving from said last load impedance rectified voltage corresponding to amplitude modulation.

3. In combination with a frequency discriminator consisting of a pair of oppositely mistuned resonant circuits, a pair of opposed diode rectiiiers each having its electrodes coupled to a respective'resonant circuit of said discriminator, a separate load impedance in circuit with each diode rectifier, a resistor connecting said load impedances in series relation, means grounding the cathode end of one of said load impedances, a gain control connection to the anode end of the series-connected second impedance for deriving from said impedances in additive polarity sense rectified direct current voltages thereacross thereby to provide a gain control voltage which is negative relative to ground, a second connection to the midpoint of said resistor for deriving a modulation voltage resulting from said rectied voltages being combined in polarity opposition.

4. In combination with a frequency discriminator, means for impressing frequency modulated signal Waves thereon, a pair of opposed diode rectiers each having an input electrode coupled to of said resistance to ground through a condenser connected to anintermediate point onone of said load impedances, a modulation frequency connection from the other end of the resistance to ground through a second condenser connected to an intermediate point on the other of said load impedances, and a connection substantially to the midpoint of said resistance for deriving a modulation voltage resulting from said rectied voltages being combined in opposition.-

5. The combination with a frequency discriminator, means for impressing frequency modulated signal Waves thereon, a pair of opposed diode rectifiers each having an input electrode coupled to said discriminator, and a separate load impedance in circuit with each of said diode rectiers for deriving a rectified voltage therefrom, the improvement comprising an output impedance, a' modulation frequency connection from one end of said output impedance to an intermediate ground point on one of said load impedances, a modulation frequency connection 'from the other end of the output impedance to an intermediate point on the other of said load impedances, and a connection substantially to the midpoint of said output impedance for deriving a modulation voltage resulting from said rectied voltages being combined in opposition.

6. The combination with a frequency discriminator,- means for impressing-frequency modulated signal Waves thereon, a pair of opposed diode rectiers each having an input electrode coupled to said discriminator, and a separate load impedance in circuit with each of said diode rectiers for deriving a rectified voltage therefrom, all of which are known; of an output impedance, a modulation frequency-connection from one end of said output impedance to ground through a portion of one of said load impedances, a modulation frequency connection from the other end of the output impedance to ground through a portion of the other of said load impedanceaa second output impedance, a modulation frequency connection from one end of said second output impedance to ground through another portion of said one of said load impedances, a-modulation frequency connection from the other end of the second output impedance to ground through another portion of the other of said load impedances, and connectionsl substantially to the midpoints of said output impedances for deriving push-pull modulation voltages from said rectied voltages.

'7. The combination with a frequency discriminator, means for impressing frequency modulated signal Waves thereon, a pair of opposed diode rectifiers each having an input electrode coupled to said discriminator, and separate, substantially equal load impedances in circuit respectively With each of said diode rectiers for deriving a rectiiied voltage therefrom, condensers of low impedance to modulation frequencies respectively connecting substantially the midpoints of the load impedances to ground, output impedances connected to different terminals of the load impedances and each to terminals of opposite modulation voltage, and connections substantially to the midpoints of said output impedances for deriving push-pull modulation voltages from said rectied voltages.-

8. The combination with a frequency discriminator, means for impressing frequency modulated equal load impedances in circuit respectively with each of said diode rectifiers for deriving a rectii'ied` voltage therefrom, connections having low impedance to modulation frequencies between substantially the midpoints of the load impedances and ground, output impedances connected to diierent terminals of the load impedances and each to terminals of opposite modulation voltage, connections substantially to the midpoints of said output impedances for deriving push-pull modulation voltages from said rectied voltages, and means for deriving automatic volume control voltage from said load impedances in aiding phase.

9. In combination with a frequency discriminator, a pair of opposed rectiiiers each having an input electrode coupled to said discriminator, a separate load impedance in circuit with each reotier, a rst resistive means connecting a rst pair of points of opposite polarity of said load impedances, a second Aresistive means connecting a second pair of points of opposite polarity of said impedances, and a separate output connection to an intermediate point on 4each of said two resistive means for push-pull modulation Voltage resulting from said rectified voltages being combined in opposition.

10. In combination with a frequency discriminator comprising at least two resonant circuits, a pair of opposed diode rectiiiers each having an input electrode coupled to a respective resonant circuit of said discriminator, a separate load imr lated carrier wave energy coupled to one of the rectiers and including the respective load impedance in circuit therewith, one of said discriminator circuits being in circuit with said source but not aiecting rectication of said amplitude modulated Wave energy, and said output connections deriving irom said last load impedance rectified voltage corresponding to amplitude modulation.

WINFIELD R. KOCH.

REFERENCES CITED The following references are of record in the iile yof this patent:

UNITED STATES PA'I'NJ Ts Number Name Date 2,354,959 Y McCoy Aug. 1, 1944 2,097,937 Rust NOV. 2, 1937 2,286,878 Roberts June 16, 1942 

